A full-range, current-limiting fuse is one where the fuse elements can sense and clear all fault currents which range from 125% to 1000.times. the name plate rating. The element material commonly used in current-limiting fuses is silver or copper. It takes temperatures of 960.degree. centigrade to 1300.degree. centigrade to melt these metals. These element temperatures can be obtained very easily when the fault current through the fuse is greater than 6.times. the nameplate rating.
In order to make the elements melt when the fault current is below 6x nameplate rating, several methods are used or have been used. One of the most common methods is to put a drop of solder on the midpoint of the element's length. As the fault current flows through the element, it raises the temperature of the element to a temperature above the melt temperature of the drop of solder which is much lower than the melt temperature of the element. The solder will melt and metallurgically react with the element material to cause it to melt open. This method is hard to control and can degrade the fuse element over a long period of time, causing it to open when it shouldn't. Another problem is that the resulting time-current curve of such a fuse element has an undesirable shape in that it is hard to coordinate with the other protective devices on the electrical system. Good coordination is when the time current curve of one element will lay alongside that of another element without the two curves touching or intersecting each other.
In some designs, the main fuse element is connected in series with a secondary fuse element of different design and alloy. The main element, composed of a high melt temperature alloy, controls the high-fault current interruption part of the time current curve and the secondary element, composed of a lower melt temperature alloy, controls the lower magnitude fault-current interruption part of the curve. This combination results in a curve that provides good high fault-low fault current interruption coordination but leaves some areas unacceptable. Low-fault sensing is generally achieved through the use of a series-connected fuse element composed of a eutectic alloy.